With the progress in miniaturization of patterns in these days, EUV (Extreme Ultra Violet) having a shorter wavelength than that of ArF has been used in a lithography for use in manufacturing semiconductor devices (semiconductor integrated circuits). By shortening the wavelength of lithography light, resolution of a pattern transferred by the lithography is improved and a finer pattern can be formed; while, according as the wavelength gets shorter, the system structure and the exposure method have been changing from the conventional exposure method. A projection optical system has been drastically changed. The conventional projection optical system has used an inflection lens; in the EUV exposure, however, EUV does not pass through the inflection lens due to a relation with optical absorption and refractive index. Therefore, in the EUV lithography, the conventional inflection lens cannot be used but a reflection optical system such as a mirror has to be used.
In manufacturing a mirror, its surface has to be polished; however, it is difficult to polish it completely flat but roughness remains on the mirror surface. At a pattern exposure, this roughness irregularly reflects the lithography light emitted on the mirror surface and the scattered light falls on a not-intended region on the resist surface of a wafer. Therefore, contrast is deteriorated on the wafer and the pattern image becomes blurred, thereby causing a problem that the finished dimension (finished pattern shape) is not a desired one.
The exposure light irregularly reflected on the mirror surface is called flare, which is one of a big factor causing degradation in forming an accurate pattern using the EUV. Because the amount of flare is affected by the scattered light nearby, it varies depending on the pattern density (brightness) in the surrounding. Therefore, a problem arises that the finished dimension is different depending on a position (pattern arrangement position) even in the same mask pattern. There is needed a technology for performing a mask pattern correction with flare taken into consideration, at high speed and at high accuracy.